Origin and development of embryo and bud primordia during maturation of embryogenic calli of Zea mays

Abstract

Maize genotype 4C1 calli under embryo maturation conditions give rise to somatic embryos that can be regenerated into plants. This scanning electron microscope study shows that the pattern formation is as in zygotic embryos. A globular stage somatic embryo forms a starch-containing scutellum, a coleoptile, and leaf primordia in the same order and pattern as in zygotic embryos. There is no callus tissue between shoot and root meristems, as occurs in organogenesis. However, the study also reveals a different order of events: shoot meristems with leaf primordia develop at the base of leafy structures, on the surface of the callus. Similar structures were often named somatic embryos in the plant tissue culture literature, but they are more comparable to axillary buds. A coleoptile is not present. Both structures develop into plants on regeneration medium. Maturation of somatic embryos that are attached to callus aggregates is impaired in liquid culture; fewer embryos mature and regenerate probably because their cells have less cell to cell contact than those grown on solid medium. Somatic embryos unattached to callus tissue grow in suspension culture; they lack a scutellum or have only a rudimentary one, lack a coleoptile and leaf primordia. They develop roots but do not develop into plants in the media used in this study. Key words: in vitro culture, pattern formation, scanning electron microscopy, somatic embryogenesis, suspension culture, Zea mays L.